This invention relates to an imaging system module comprising high density avalanche chamber (HIDAC) converters and in particular to an imaging system for use in positron emission tomography (PET).
It is known to provide a HIDAC for use in PET. IEEE Tran. Nucl. Sci. NS30 640 (1983) describes the construction of one such form of HIDAC. U.S. Pat. No. 5,434,468 also discloses a HIDAC for use in imaging of beta radiation.
The HIDAC of U.S. Pat. No. 5,434,436 includes a gas tight, radiation transparent enclosure that may be filled with an inert gas during sampling. The incidence of beta radiation on the inert perforations of the converter ionizes the gas. Products of the ionization (typically electrons) are avalanched in the perforations and extracted towards the planar anode by high biasing voltages applied to the converter. Contact with the anode causes further avalanching and current pulses in the x- and y-axis components of the cathodes. Analysis of the cathode currents by signal processing circuits enables imaging of the radiation source.
In addition to the foregoing, it is known to provide a modified form of HIDAC suitable for imaging of gamma radiation sources. Such a HIDAC includes lead, which is stimulated to emit photoelectrons when subjected to gamma radiation, in order to compensate for the inability of gamma radiation directly to ionise the inert gas.
It is also known to provide a stack of converters of the types described above to increase the detection efficiency.
Although the apparatus described above has provided significant advances in the field of radiation imaging there remains a need for more efficient apparatus and particularly for apparatus which provides a reduction in the time taken to form an image.
According to a first aspect of the invention, there is provided an imaging system module comprising: a pair of high density avalanche chamber converters, each converter including a series of alternate layers of conducting and non-conducting material and an array of parallel, through-going apertures extending through said series of alternate layers, a first converter of the pair having a plurality of conducting elements extending generally parallel to each other in a first direction to form a first cathode on or adjacent to a face of the first converter and the second converter of the pair having a plurality of conducting elements extending generally parallel to each other in a direction generally orthogonal to the first direction to form a second cathode on or adjacent to a face of the second converter, and an anode formed by a series of generally parallel conducting elements positioned between the first and second cathodes, the arrangement being such that radiation incident upon either converter produces an avalanche of charged particles which are attracted towards the said anode and the incidence of a charged particle on the anode causes a current pulse in both the first and second cathodes.
According to a second aspect of the invention, there is provided an imaging system comprising a pair of detectors, each comprising a module as detailed above, the detectors being positioned opposite each other so that a radiation source of which an image is to be formed can be positioned therebetween.
According to another aspect of the invention there is provided PET apparatus incorporating one or more imaging system modules or an imaging system as described above.
Other preferred and optional features of the invention will be apparent from the following description and from the subsidiary claims of the specification.